Fuel-injection for direct-injection diesel engine

ABSTRACT

In order to reduce soot emissions from diesel engines, the necessary mixing of air and gasified fuel must be completed prior to the main combustion. On the other hand, noise is reduced to a minimum by providing that the pressure increase per degree of crankshaft rotation during combustion is no greater than the maximum pressure increase during the compression stroke.

Fuel injection in piston combustion machines using a plurality ofinjection nozzles is generally employed for the introduction ofdifferent fuels, for varying the injection quantity by opening andclosing the different nozzles and for uniformly distributing the fuel inthe combustion chamber.

The present application is concerned with the pressure/temperaturecharacteristics and the injection conditions which are to be achievedfor a direct-injection diesel engine in order to reduce importantemissions. A method of operating a diesel engine is proposed and isbased on the duothermal process since it is on the basis of this processthat reductions in emissions are measured in actual experiments. Itshould be possible to carry over the knowledge and teachings which areobtained to other applications and procedures.

The increasingly restrictive regulatory emission standards forcombustion machines affect diesel engine operation, especially asregards noise and soot particles. The relationship between these twotypes of emissions will be explained below. With respect to sootparticle emissions, problems arise particularly for fuels which aredifficult to vaporize. Accordingly, the method steps to be outlined wereconceived and performed experimentally for fuels, including vegetablefats, which are difficult to vaporize. For vegetable fats, specifically,it has been found that a series of advantages are obtained once theproblem of fuel vaporization is overcome since these developing fuelshave a neutral CO₂ balance and do not produce SO₂ or benzol (derivative)emissions.

For diesel engine operation, the object is thus to find a solution whichachieves a reduction in soot particle emissions and also limits noise.

In the combustion method which is now provided as a solution, the numberof injection nozzles with throttling pins is multiplied so that two ormore such nozzles are used with the aim of shortening the mixingprocess, and hence the injection time. The need for accelerating themixing process is due to the following: Soot particles are formedbecause liquid fuel continues to be brought into the combustion chambereven when the combustion procedure is terminating, that is, theconventional injection and mixing times cause liquid, gaseous andburning fuel components to exist concurrently over an extended period.This brings the danger that, to the extent the liquid fuel brought intothe ongoing combustion process consists of hydrocarbons, only hydrogencan still be split off in the delayed gas phase while the carbon becomesvisible as uncombusted soot.

As a first method step, then, an increase in the mixing speed and areduction in the injection time are required so that, during injection,the last quantity of liquid fuel brought in vaporizes before the fueladmitted previously has generated such a high combustion temperaturethat the danger of soot formation exists for the last quantity of fuel.If all of the fuel is in the gas phase before the combustiontemperatures become too high, the problem of soot formation iseliminated. When the mixing speed of air and fuel is increased while, onthe other hand, the injection time is reduced, a substantial temperatureincrease of the working air during the injection period is avoided.

If, in addition, as the next important method condition, the combustionzone is concentrated at the center of the existing combustion chamber,the use of a plurality of injection nozzles enables the thus-acceleratedmixing procedure to be fully utilized and a very rapid vaporization ofthe fuel to be achieved In order to prevent the liquid fuel from cominginto contact with the combustion zone, it has already been proposed tocarry out fuel vaporization at the wall of the combustion chamber, thatis, for the fuel to be deposited on the wall of the combustion chamberas a film and to be transferred from there to the combustion chamber airand the combustion zone in vapor form. Although the liquid fuel heredoes not come into contact with the burning gases, increased CHemissions in the form of aldehydes, etc., instead of soot formation,occur in this process.

The two steps of increasing the mixing speed of air and fuel and ofdeveloping a central combustion zone surrounded by air are thus methodconditions. Particular emphasis must be placed on achieving the smallestpossible proportion of air since a stoichiometric mixture of air andfuel does not leave any excess oxygen for undesired NO_(x) formation.The optimal concentration corresponding to the smallest possibleproportion of air is obtained by separating combustion air and excessair. In the resulting duothermal process, the excess air and combustionair are subjected to intense rotation (see West German Patent No. 22 41355, British Patent Specification No. 2 000 222 and West GermanOffenlegungsschrift No. 33 43 677 for this process).

The rotation of the combustion chamber air is so intense that thequantities of fuel injected inwards from the combustion chamber walltowards the combustion zone are deflected before they reach the oppositeside of the combustion chamber. The fuel is thus maintained insuspension in the hottest portion of the combustion chamber air until ithas vaporized. The accuracy of this procedure must be increased as thedifficulty of vaporizing the fuel increases. This is the case withvegetable fats. However, if vegetable fat is totally vaporized prior tothe main combustion, the advantages of the vegetable fat as regards themolecular structure of the hydrocarbons during the subsequent combustionprocess can be fully realized. The chain-like arrangement of thehydrocarbons is exceedingly suitable for smokeless combustion.

The pressure increase during the combustion process is the third methodcondition to be taken into account. With the rapid mixing which isachieved using a plurality of throttling pin nozzles and the rapidvaporization which occurs in the isolated duothermal combustion zone,the pressure increase in a cylinder during combustion should notgenerate undue noise. The pressure increase per degree of crankshaftrotation (Δp/Δα) should not exceed a predetermined value. Low noiselevel and freedom from soot are currently equally fundamental andimportant for the utility of the diesel engine Accordingly, when viewingthe two problems together, it is necessary to define a goal, other thanthe goals of accelerated injection and accelerated mixing of air andfuel, which causes the method not only to be effective but also togenerate little noise.

Unacceptable noise generation can be avoided by late injection whichallows the rate of pressure increase during combustion to be held to themaximum value of the compression phase. The conventional formula fordiesel engine combustion by which pressure remains unchanged duringcombustion thus changes to the extent that the rate of pressure increaseduring firing of the cylinder, and not the pressure in the cylinder,remains constant. The constant rate of pressure increase during firingof the cylinder, which corresponds to the maximum pressure increase ofthe compression stroke, is maintained for the duration of the combustionprocess by virtue of the fact that the large quantities of fuel admittedby the plurality of nozzles burn increasingly rapidly therebycompensating for the reduction in pressure increase during the expansionstroke of the piston. Mixing must be accelerated by means of anappropriate number of nozzles as retardation of injection increases.

The constant pressure increase, that is, the regulation of injectiontime and injection quantity, is controlled via the characteristics ofthe throttling pin nozzles using pre-injection, if necessary, as isknown in the art.

Intensive cooling of the air charge enhances this method since it isgood to develop a large temperature differential between the combustionzone air and the excess air. This also helps to suppress NO_(x)formation. The delayed start of injection in the method further helps toreduce NO_(x).

In order to increase the number of possible combinations of injectionpressure and injection time, a transverse bore is, for the first time,formed in the cylinder head. This bore is so large that an injectionline for a second injection nozzle which is offset by 180° can extendthrough the same. Such measure is made possible by using a cylinder headfor the present method which is cooled via a few bores by the engine oilonly. By eliminating the cooling chambers which are normally providedfor water cooling, it becomes possible to implement the above proposalinvolving the bore for the injection line.

FIGS. 1 and 2 illustrate embodiments with one and two nozzles. Each is aplan view of the combustion chamber of a direct injector.

FIG. 3 is a plot of cylinder pressure versus crankshaft rotationalangle.

FIG. 4 is a plot of cylinder pressure per degree of rotation versuscrankshaft rotation angle.

FIG. 5 shows cylinder head for two injection nozzles.

FIG. 1 represents the prior art as regards the duothermal combustionprocess. The air in the combustion chamber 1 and 2, which air is in theform of a vortex, is bounded by the combustion chamber wall 3. Afterinjection by means of an injection pin nozzle 4, the spray from whichdoes not reach the opposite side because of the intense vortex, twozones 1 and 2 are formed. Zone 1 constitutes the combustion zone with astoichiometric air/fuel mixture. Zone 2 contains excess air which doesnot take part in the combustion and functions as an insulating jacketbetween the hot combustion zone and the cool combustion chamber wall 3.

As an example of the use of a plurality of injection pin nozzles, FIG. 2illustrates the case for two such nozzles 5 and 6. The flow situation ismaintained; the duothermal arrangement of the combustion zone 1 and theexcess air 2 is formed as in FIG. 1 since the fuel injected by the twonozzles does not reach the combustion chamber wall 3. FIG. 2 also showsthat the entire combustion zone has been supplied with fuel while, inFIG. 1, only half of the mixing procedure is complete within the sametime span and for the same rotational speed of the air vortex. Expresseddifferently, the mixing time is halved when the number of nozzles isdoubled, is reduced to one-third for three nozzles, etc.

In the diagram of FIG. 3, the ordinate represents the pressure in thecylinder and the abscissa the angle of crankshaft rotation. The ordinatein FIG. 4 represents the pressure in the cylinder per degree ofcrankshaft rotation while the abscissa represents the angle ofcrankshaft rotation. The pressure variation which can be obtained in thecylinder with a plurality of injection nozzles and is desirable from theviewpoint of noise is illustrated in FIG. 3 and, between point 12 andpoint 13, corresponds to a line shown in FIG. 4 and representing aconstant pressure increase, Δp/Δα, of 2.8 bars per degree of crankshaftrotation. The curve 8 in FIG. 3 shows the pressure variation when theengine is not firing while the curve 9 in FIG. 4 illustrates thepressure increase when the engine is not firing.

The substance of the pressure variation according to the invention isrepresented by the line 10 in FIG. 3 and the line 11 in FIG. 4 extendingbetween the points 12 and 13. The pressure increase from point 12 topoint 13 is approximately constant. The line from point 13 to point 14corresponds to the dying out of combustion and is followed by thetheoretical expansion line 15.

FIG. 5 shows a cylinder head 16 for two injection nozzles 17 and 18. Thecylinder head is bounded from below by the cylinder 21 and from above bythe cylinder head cover 22. The fuel line 19 leads directly to thenozzle while the fuel line 20 for the second nozzle extends through abore in the cylinder head.

We claim:
 1. A method of operating a combustion engine, particularly adirect-injection diesel engine, having a combustion chamber, comprisingthe steps of admitting air into said chamber; admitting fuel into saidchamber, mixing said fuel with at least part of said air to form acombustible mixture; compressing said air in said chamber; combustingsaid mixture to generate power, the pressure in said chamber increasingduring the combusting step; rotating a crankshaft with the powergenerated during the combusting step; and maintaining the pressureincrease per degree of crankshaft rotation during the combusting step atleast approximately equal to the maximum pressure increase during thecompressing step.
 2. The method of claim 1, wherein the air admittingstep comprises imparting a circulatory motion to said air so that saidair forms a vortex in said chamber, said vortex having a central portionand a peripheral portion surrounding said central portion, and themixing step including mixing said fuel with the air in said centralportion to form an at least approximately stoichiometric combustiblemixture.
 3. The method of claim 2, wherein the fuel admitting stepcomprises admitting at least two streams of fuel into said chamber. 4.The method of claim 1, said chamber being bounded by a peripheral wall,and said fuel being in a non-gaseous state upon admission into saidchamber; and further comprising the steps of converting said fuel to agaseous state, and maintaining said fuel out of contact with saidperipheral wall until the converting step is substantially complete. 5.The method of claim 4, wherein said fuel is of vegetable origin.
 6. Themethod of claim 4, wherein the step of maintaining said fuel out ofcontact with said peripheral wall comprises maintaining said fuel insuspension.
 7. The method of claim 1, wherein said fuel is in anon-gaseous state upon admission into said chamber; and furthercomprising the step of converting said fuel to a gaseous state beforethe temperature in said chamber reaches a value at which combustion ofsaid fuel generates carbonaceous particles.
 8. The method of claim 7,wherein said fuel is of vegetable origin.
 9. The method of claim 1,wherein the fuel admitting step comprises admitting at least two streamsof fuel into said chamber through respective apertures; and furthercomprising the step of regulating the cross-sectional area of at leastone of said apertures.
 10. The method of claim 1, wherein the fueladmitting step comprises admitting at least two streams of fuel intosaid chamber through respective apertures, the admitting step for one ofsaid streams being initiated prior to the admitting step for the otherof said streams.
 11. The method of claim 10, wherein the step ofmaintaining the pressure increase comprises regulating thecross-sectional area of at least one of said apertures.
 12. The methodof claim 10, wherein the fuel admitting step comprises conveying saidfuel from a fuel source to said apertures along respective paths, one ofsaid paths being longer than the other of said paths.
 13. The method ofclaim 12, wherein said engine includes a cylinder head which bounds saidchamber and the fuel admitting step comprises conveying fuel to at leastone of said apertures via said cylinder head.
 14. The method of claim13, wherein said apertures are disposed at substantially diametricallyopposite sides of said chamber and the fuel admitting step comprisesconveying fuel to said one aperture by passing such fuel across saidchamber through said cylinder head.
 15. The method of claim 1, whereinthe air admitting step comprises imparting a circulatory motion to saidair so that said air forms a vortex in said chamber, said vortex havinga central portion containing air in an amount at least approximatingthat required to form a stoichiometric combustible mixture with saidfuel, and said vortex further having a peripheral portion whichsurrounds said central portion and contains excess air; and furthercomprising the step of establishing a temperature differential betweensaid central portion and said peripheral portion, the establishing stepincluding cooling at least part of said air.
 16. The method of claim 1,further comprising the steps of lubricating said engine with alubricant; and cooling said engine by circulating a cooling fluidtherethrough, said cooling fluid being constituted substantiallyexclusively by said lubricant.